Several types of luxmeters are known to those skilled in the art. A first type contains a photoconductive light sensor. The problem with this solution is the non-linearity of the voltage across the sensor terminals as a function of the light received. Another type uses a photodiode or phototransistor.
In a system incorporating a photovoltaic cell as the electrical energy source, it is advantageous to avoid adding a specific sensor for measuring light intensity. Indeed, it is known to use a photovoltaic cell in a luxmeter. The problem is to perform such a measurement accurately and without diminishing the charging efficiency of an element for storing the electrical energy provided by the photovoltaic cell. A relatively simple solution would be to measure the open-circuit voltage (output voltage at which the photovoltaic cell delivers no current), but such a solution gives an inaccurate result since the open-circuit voltage varies little as a function of the received luminous intensity and moreover this variation is not linear as a function of luminous intensity.
A luminous intensity measurement via the photovoltaic cell raises a technical problem, given the conditions of a system performing with one or more photovoltaic cells. Such a system includes a storage element (storage capacitor or battery) connected to the output terminal of the photovoltaic cell via a DC-DC converter having an external input capacitor arranged in parallel. This external capacitor generally has a relatively high value, notably several microfarads. It is necessary for the proper efficiency of the converter. Next, the solar cell can generally generate an electrical current over a wide range, notably from several microamperes to several milliamperes. Further, in order to perform a luminous intensity measurement, it is preferable for the measurement to be made in quite a short time interval, for example less than 100 milliseconds (100 ms).